The ring laser gyros taught by the prior art have used epoxy for hermetic bonding of components. Although this method is simple to effect, it has been found that epoxy does not provide a suitable seal for ring laser gyros because of high outgassing and water vapor permeability and it is no longer commonly applied in this environment.
At present, the most commonly used method of hermetic bonding is by vacuuming soldering of premetallized surfaces. This method was first described by U. Hochuli in the Review of Scientific Instruments (Volume 43, No. 8, August 1972). The interface surfaces of the components to be hermetically bonded are first metallized with a thin film of gold. This is usually done by high temperature air firing of a liquid gold resin sprayed on the surface. The components are then held together and placed in a high vacuum oven where the entire instrument is heated to approximately 200.degree. C. Indium solder is then allowed to flow into the seal interface dissolving the gold film and forming a bond.
The method of bonding seal in building ring laser gyros (RLG's) taught by the prior art has several drawbacks. It requires very involved surface preparation, a high temperature gold firing process, involved vacuum oven soldering, and the requirement of elaborate fixturing. It has been found that the reliability of the process is relatively poor. The thin film fired-on gold metallization process is quite difficult to control in that many materials and other factors determine the outcome. Among problem areas are: difficult control of coating thickness (too much thickness results in flaking) and too little thickness results in a poor seal. It has been found that at least two layers of gold are required for a good seal. Even then, the outcome depends on the type of thinner, eveness of coat, age and condition of the gold resin, drying temperature, firing temperature, rate of temperature increase, hold time, smoothness of surface, and oxygen contents in the firing oven. Some materials, such as Pyrex, don't tolerate the high firing temperature of approximately 700.degree. C. that is necessary. The resulting gold film is highly susceptible to scratching during handling. This causes leaks in the resulting seal making it necessary to repeat the entire gold coating/indium sealing process. Also, this process cannot bond materials such as aluminum to glass ceramics. This is because of the well-known problem with intermetallics (purple plague) that prevents gold to aluminum bonds.
A. Roth describes in "Vacuum Sealing Techniques", page 329 (1966 Pergamon Press) the use of indium wire as a gasket held in constant compression in hermetic sealing. In this form, the indium served the same function as a rubber gasket in that it provides a sealing medium with no bonding properties. The parts are held together with bolts and/or spring clamps, i.e., a standard compression flange design.
In U.S. Pat. No. 3,951,326, Snow et al teaches another bonded seal method. The method taught therein makes use of the well-known fact that aluminum compression seals when heated during compression will bond to seal surfaces. Snow et al apply this seal method to a microcircuit package. The method taught by Snow et al would not be applicable to RLG's seals because of the high pressure required as well as the high temperature requirement. Although Snow et al mentions copper, platinum, gold, silver and titanium as seal materials in their device, indium and its alloys are not taught by them. Nor do Snow et al teach plasma cleaning and the requirement for a smooth surface finish on the parts to be bonded.